The invention relates to a device for welding metal elements, in particular studs, to structural parts, having a welding head that has a collet chuck for the accommodation of an element, having a feed unit for the feed of elements into the chuck, and having a power supply means for the supply of electric energy for a welding procedure.
A welding device of this type is disclosed for example in EP 0,784,526 B1. Here the welding process used is so-called short-time arc welding, which is also generally known by the term “stud welding.” In this process, an element accommodated in a collet chuck in connection with a welding procedure is first raised with respect to the structural part, a welding arc is formed between the element and the structural part and the element is then lowered again.
The technology of stud welding is used especially, but not exclusively, in the automotive industry. This technology allows studs, with or without thread, nuts, eyes and other elements to be welded to metal sheets of car bodies. The elements then as a rule act as holding anchors for fastening interior fittings to the body of the vehicle.
In the stud welding mentioned above, an element first is inserted into a collet chuck of a welding head. This may be effected by an automatic feed means, for example by means of compressed air. After suitable positioning of the element with regard to the structural part, as a rule by means of a robot arm, the welding procedure is begun.
It goes without saying that with such welding devices a multiplicity of welding procedures must be carried out successively, as a rule fully automated. Here, the elements to be welded may for example be inserted directly into the chuck by means of compressed air. Such automatic feed generally can be reliably achieved. At the same time, it goes without saying that a welding procedure should be begun only when an element to be welded is actually located in the chuck. In continuous operation, however, failures in automatic feed of elements into the chuck may also occur. In such cases, failure in the feed of elements to be welded should be detected as early as possible and, for safety reasons, no attempt should be made to commence a welding procedure.
The object of the invention therefore is to procure an improved device for the welding of metal elements to structural parts, by which a failure in the automatic feed of metal elements into the collet chuck can be detected as reliably as possible. This object is accomplished in a device of the type mentioned at the beginning in that the collet chuck has at least two segments electrically isolated from one another. In this way, the object of the invention is fully accomplished. Namely, here examination, directly on the chuck, of whether or not an element to be welded is located in the chuck is made possible in extremely simple and low-cost fashion. For this purpose, only the electrical connection that is produced between the segments isolated from one another upon receipt of a metal element need be evaluated.
In an advantageous refinement of the invention, the segments electrically isolated from one another are coupled with a sensor means for the detection of an electrical contact being made between the electrically isolated segments. For this purpose, according to another embodiment of the invention, the segments may be connected with a test voltage and a flow of current between the segments may be evaluated by the sensor means. Here the sensor means may generate a signal, preferably digital, which indicates whether an element is carried in the chuck.
According to another embodiment of the invention, a control means, with which the sensor means is coupled, is provided for control of the welding procedure. These measures make it possible, in especially simple, low-cost and reliable fashion, to determine whether or not an element to be welded is located in the chuck. By evaluation of the flow of current and advantageously its conversion into a digital signal, the control means, which as a rule is a microprocessor control, therefore can then emit a digital signal that indicates whether an element to be welded is located in the chuck or not. The control unit may be programmed in such a way that a welding procedure can be started only when it is detected that an element to be welded is held in the chuck. Otherwise, an appropriate error signal optionally may be emitted.
According to a first variant of the invention, the collet chuck has segments designed flexible. Here the chuck may have two segments, which, as customary in conventional one-piece collet chucks, are segmented into unbroken subsegments by longitudinal slits.
According to a second variant of the invention, the collet chuck has guide sections on its segments for the radial feed of elements. Such a design is advantageous especially when the elements to be welded are not to be fed into the chuck axially, but rather, laterally. Here, for the feed of an element to be welded, the two segments are moved apart and after feed of the element to be welded are moved together again until the element is held between the two segments of the chuck. The guide sections may alternatively be designed springy, in order to facilitate radial feed. Here, electrical evaluation of the test voltage via the sensor means may simultaneously be utilized for position control for correct closing motion of the two segments of the chuck, which represents an additional advantage.
According to an additional embodiment of the invention, the segments, isolated from one another, may be held together by an annular element. This is advisable especially in the embodiment as a flexible collet chuck, for which for example an open ring of synthetic material may be used. It goes without saying that the features of the invention mentioned above and those yet to be explained below may be used not only in the combination indicated in each instance, but alternatively in other combinations or standing alone, without departing from the scope of the invention. Additional features and advantages of the invention appear in the following description of preferred examples with reference to the drawing, wherein: